1. Field of the Invention
The present invention generally relates to a photolithographic process, and more particularly, to a prediction model and a prediction method for exposure dose.
2. Description of Related Art
In general, the basic parameter that affects the critical dimension (CD) of a photolithographic process is mainly the exposure dose. Conventionally, the control of the critical dimension of a photolithographic process is carried out after the exposure process. According to the result of the last measured critical dimension, compensation to the exposure dose is provided. Moreover, a preset exposure dose is given out through the dose setting system of the exposure machine so that the critical dimension value for the next lot of wafer is closer to the target value. The exposure dose can be measured by using a measuring machine to measure the photoresist pattern on a production wafer and using the measured result to provide an automatic feedback to the exposure machine. For example, an advanced process control (APC) method can be used such that the exposure machine operates under the control of ideal parameters.
However, the conventional method cannot and does not take into consideration the difference of the critical dimension of the mask and the deviation of critical dimension of line product from a baseline. Therefore, the critical dimension of the final exposure pattern often deviates from a target value or even exceeds the tolerable deviation so that the exposure conditions of the product must be modified anew. In other words, a reworking step of the exposure process must be executed. The so-called ‘difference in the critical dimension of the mask’ refers to the difference between the critical dimension of the initial design (that is, the target value) and the critical dimension actually fabricated on the mask.
Accordingly, when the semiconductor device is manufactured in the deep sub-micron dimensional range, dimension of the device is so small that the acceptable tolerance of the dimension of the designed device is also smaller. Under such circumstances, the photolithographic process is challenged. For example, considering the effect of the difference of the critical dimension of the mask on the pattern dimension that needs to be transfer and accurately finding the most appropriate exposure dose for manufacturing a device with acceptable tolerance is one of the targets of development for most manufacturers.